Micro bump and method for forming the same

ABSTRACT

A method for forming a micro bump includes forming a first nano-particle layer on a substrate and forming a second nano-particle layer on the first nano-particle layer. The first and second nano-particle layers include a plurality of first nano particles and a plurality of second nano particles, respectively. The method further includes irradiating a laser beam onto the second nano-particle layer, where the laser beam penetrates through the second nano-particle layer and is at least partially absorbed by at least some of the first nano particles to generate heat. The first nano particles and the second nano particles have different absorption rates with respect to the laser beam.

RELATED APPLICATION

This is a division of application Ser. No. 13/098,209, filed Apr. 29,2011, the entire contents of which are incorporated herein by reference.

TECHNOLOGY FIELD

The present disclosure relates to semiconductor device manufacturingand, more particularly, to a micro bump, a method for forming the microbump, and a package comprising the micro bump.

BACKGROUND

With the development of semiconductor technology, three-dimensional (3D)packaging has become more widely used. An integrated circuit employing3D packaging technology may be called a 3D-IC. In a 3D-IC, chips may bevertically stacked on top of each other, with different chips beinginterconnected using interconnects such as through-silicon-vias (TSVs),bumps, and/or redistribution layers.

Among different kinds of 3D-ICs, those having logic chips and memorychips stacked in one package may be challenging to manufacture. Sincethere are many TSVs in such a 3D-IC, a large number of bumps may also berequired to connect the TSVs in one chip and wirings on another chip.However, since chip area may be limited, the large number of bumps mayrequire reducing a horizontal size of the bumps, such as a diameter of acircular bump, a side length of a square bump, or a length of a shortside of a rectangular bump. The horizontal size of the bumps may need tobe reduced to about 10 μm or even smaller. The traditional method ofmanufacturing lead-free solder bumps by a printing process may not besuitable for manufacturing bumps of such size.

Generally, a 3D-IC packaging process may require low temperature (suchas a temperature lower than about 200° C.), low pressure (such as apressure lower than 10 MPa), and non-vacuum condition during bonding oftwo chips. Moreover, bumps between the chips may need to have highstrength and low resistivity. Currently, there are two categories ofbonding methods that may be used for 3D-IC packaging. One is solderbonding, and the other is thermocompression bonding using copper bumps.However, the existing techniques cannot achieve a bump having ahorizontal size of about 10 μm or smaller, which may be used to bond twochips at a bonding temperature lower than 200° C. and a bonding pressuresmaller than 10 MPa in a non-vacuum environment. For example, use ofSnAgCu solder can only achieve a bump having a horizontal size largerthan 25 μm. Solder bonding using CuSn solder may be used to manufacturebumps having smaller size, but the temperature required for bondingusing CuSn solder may need to be higher than 250° C. Thethermocompression bonding method using copper bumps may need a highertemperature of about 400° C., a pressure higher than 10 MPa, and avacuum environment. Also, a 3D-IC packaged using thermocompressionbonding method may have large stresses built up in the chips. This maybe especially true for an IC package composed of chips with smallthickness and the stresses may cause the chips to crack.

Recently, metal nano particles have been employed as bonding materialfor microelectromechanical systems (MEMS), surface mount diodes (SMD),and light emitting devices (LED). Due to their small size, metal nanoparticles may have a low melting temperature, so that the bondingprocess using metal nano particles may be performed at a lowtemperature. However, in most existing methods, metal nano particles arecoated on chips in a form of paste or ink, These methods may not be ableto form micro bumps having a horizontal size smaller than 10 μm, andthus may also not be suitable for the fabrication of a 3D-IC includinglogic and memory chips.

Further, since space exists between metal nano particles due to, forexample, non-uniform nano-particle size and protective agents such aspolyvinylpyrrolidone (PVP), if the metal nano particles are directlysubjected to a bonding process, voids may appear in the bumps so formed.For micro bumps having a horizontal size smaller than 10 μm, such voidsmay undesirably decrease bond strength. The resistivity of the microbumps may also increase due to the voids. To prevent voids from forming,the metal nano particles may be first melted and solidified, and thensubjected to the bonding process. However, in this method, since themelting/solidifying process may cause formation of larger crystalgrains, the temperature required for bonding may be increased.

SUMMARY

In accordance with the present disclosure, there is provided a methodfor forming a micro bump including forming a first nano-particle layeron a substrate and forming a second nano-particle layer on the firstnano-particle layer. The first and second nano-particle layers include aplurality of first nano particles and a plurality of second nanoparticles, respectively. The method further includes irradiating a laserbeam onto the second nano-particle layer, the laser beam penetratingthrough the second nano-particle layer and being at least partiallyabsorbed by at least some of the first nano particles to generate heat.The first nano particles and the second nano particles have differentabsorption rates with respect to the laser beam.

Also in accordance with the present disclosure, there is provided amethod for forming a micro bump including forming a first nano-particlelayer on a substrate, patterning the first nano-particle layer to form aplurality of adhesion pads, and forming a second nano-particle layerover the adhesion pads and the substrate. The first and secondnano-particle layers include a plurality of first nano particles and aplurality of second nano particles, respectively. The method furtherincludes irradiating a laser beam onto the second nano-particle layer,the laser beam penetrating the second nano-particle layer and being atleast partially absorbed by the first nano particles in the adhesionpads to generate heat. The first nano particles and the second nanoparticles have different absorption rates with respect to the laserbeam.

Also in accordance with the present disclosure, there is provided amicro bump including an adhesion layer formed of a first metal and abump layer formed on the adhesion layer. The bump layer includes aplurality of nano particles formed of a second metal and a fillingmaterial filling space between the nano particles, the filling materialbeing formed of the first metal. A weight ratio of the filling materialto the nano particles decreases from an interface between the bump layerand the adhesion layer to a top surface of the bump layer.

Also in accordance with the present disclosure, there is provided apackage including a first substrate comprising a first electrode, asecond substrate comprising a second electrode, a first adhesion layerformed on the first substrate, and a second adhesion layer formed on thesecond substrate. The first and second adhesion layers are formed of afirst metal. The package also includes a bump layer formed between thefirst adhesion layer and the second adhesion layer. The bump layerincludes a bump material and a filling material filling space in thebump material. The bump material is formed of a second metal and thefilling material is formed of the first metal. A weight ratio of thefilling material to the bump material decreases from an interfacebetween the bump layer and the first adhesion layer to a middle of thebump layer, and increases from the middle of the bump layer to aninterface between the bump layer and the second adhesion layer.

Features and advantages consistent with the present disclosure will beset forth in part in the description which follows, and in part will beobvious from the description, or may be learned by practice of thepresent disclosure. Such features and advantages will be realized andattained by means of the elements and combinations particularly pointedout in the appended claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention and together with the description, serve to explain theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E are schematic cross-sectional views showing a method forforming micro bumps according to embodiments consistent with the presentdisclosure.

FIGS. 2A-2F are schematic cross-sectional views showing another methodfor forming micro bumps according to embodiments consistent with thepresent disclosure.

FIGS. 3A and 3B are a schematic cross-sectional view and a schematicplan view, respectively, showing micro bumps according to embodimentsconsistent with the present disclosure.

FIG. 4 is a schematic cross-sectional view showing a package accordingto embodiments consistent with the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

Embodiments consistent with the present disclosure include methods forforming micro bumps, micro bumps so formed, and electronic packageshaving such micro bumps.

Hereinafter, embodiments consistent with the present disclosure will bedescribed with reference to drawings. Wherever possible, the samereference numbers will be used throughout the drawings to refer to thesame or like parts.

FIGS. 1A-1E schematically show a method for forming micro bumpsconsistent with embodiments of the present disclosure. FIG. 1A shows afirst nano-particle layer 104 formed on a substrate 102. The substrate102 may be, for example, a semiconductor substrate, a polyimidesubstrate, or a metal substrate. The first nano-particle layer 104includes a plurality of first nano particles 1042. In some embodiments,the first nano particles 1042 may have a size less than 50 nm. As usedherein, a size of a nano particle may refer to a diameter of the nanoparticle if it has an approximately spherical shape, or may refer to alength at a longest dimension of the nano particle if it has a shapeother than spherical or an irregular shape. In some embodiments, thesize of each first nano particle 1042 may be about 10 nm to about 30 nm.In some embodiments, the first nano-particle layer 104 may have athickness of about 100 nm to about 1000 nm. In some embodiments, thefirst nano-particle layer 104 may have a thickness of about 700 nm.

In some embodiments, the first nano-particle layer 104 may be formed bycoating a first solution on the substrate 102, where the first solutionincludes a solvent and a plurality of the first nano particles 1042. Insome embodiments, the solvent in the first solution may be water. Insome embodiments, the solvent in the first solution may be alcohol, suchas methanol, ethanol, isopropyl alcohol (IPA), or butanol. After thefirst solution is coated on the substrate 102, the solvent is evaporatedso that the first nano particles 1042 remain on the substrate 102 toform the first nano-particle layer 104.

Next, as shown in FIG. 1B, a second nano-particle layer 106 including aplurality of second nano particles 1062 is formed on the firstnano-particle layer 104. In some embodiments, a size of the second nanoparticles 1062 may be several nanometers to several thousands ofnanometers. In some embodiments, the size of each second nano particle1062 may be about 20 nm to about 300 nm. In some embodiments, the secondnano-particle layer 106 may have a thickness of about 1 μm to about 10μm.

In some embodiments, the second nano-particle layer 106 may be formed bycoating a second solution on the first nano-particle layer 104, wherethe second solution includes a solvent and a plurality of second nanoparticles 1062. In some embodiments, the solvent in the second solutionmay be water. In some embodiments, the solvent in the second solutionmay be alcohol, such as methanol, ethanol, isopropyl alcohol (IPA), orbutanol. After the second solution is coated on the first nano-particlelayer 104, the solvent is evaporated so that the second nano particles1062 remain on the first nano-particle layer 104 to form the secondnano-particle layer 106.

In some embodiments, since the first nano particles 1042 may alsodissolve in the solvent in the second solution containing the secondnano particles 1062, some of the deposited first nano particles 1042 maybe dissolved and mixed into the second solution. Therefore, after thesecond nano-particle layer 1062 is formed, some first nano particles1042 may be included in the second nano-particle layer 106. In someembodiments, besides the solvent and the second nano particles 1062, thesecond solution may be composed to also include a plurality of the firstnano particles 1042. In such a case, the second nano-particle layer 106would include a mixture of the first nano particles 1042 and the secondnano particles 1062.

Referring to FIG. 1C, after the second nano-particle layer 106 isformed, a laser beam 108 is irradiated onto the second nano-particlelayer 106. In some embodiments, a single laser source may be used toscan over the second nano-particle layer 106 and irradiate laser beam108 onto regions where micro bumps are to be formed. In someembodiments, an array of laser sources may be used to irradiate aplurality of laser beams 108 at the same time. For illustrativepurposes, FIG. 1C shows two laser beams 108. This should be understoodto be one laser beam irradiating at different times or two or more laserbeams irradiating at the same time.

In some embodiments, the second nano particles 1062 may not absorb thelaser beam 108 or may have a very small absorption rate of the laserbeam 108, while the first nano particles 1042 may have a largerabsorption rate of the laser beam 108 as compared to the second nanoparticles 1062. Thus, most of the laser beam 108 may penetrate throughthe second nano-particle layer 106, and be absorbed by at least some ofthe first nano particles 1042 in the first nano-particle layer 104. Thatis, at least some of the first nano particles 1042 in the firstnano-particle layer 104 may absorb at least part of the laser energy togenerate heat and thus be melted. In some embodiments, the heatgenerated by some of the first nano particles 1042 may be partiallytransferred to neighboring nano particles and cause them to melt aswell, In addition, in the case of the second nano-particle layer 106also including some first nano particles 1042, at least some of thefirst nano particles 1042 in the second nano-particle layer 106 may alsoabsorb part of the laser energy and be melted.

As used herein, the word “melt” may mean either completely melt orpartially melt. For example, the first nano particles 1042 irradiated bythe laser beam 108 may be completely melted, or only a surface portionof the particles be melted.

To selectively heat and melt the first nano particles 1042 but keep thesecond nano particles 1062 essentially unmelted, technologies such assurface plasmon resonance (SPR) absorption may be employed. In general,a metal may absorb energy of a laser beam via surface plasmon resonance(SPR) absorption. The absorption rate of the metal may depend on severalfactors such as a wavelength of the laser beam and a state of the metal(e.g., a size of the metal particles). Holding other factors constant,when the wavelength of the laser beam equals a certain wavelength, theabsorption rate of a metal in a certain state may be at its maximum.This certain wavelength is called an SPR wavelength of the metal forthat certain state. As a difference between the wavelength of the laserbeam and the SPR wavelength of the metal increases, a capability of themetal to absorb the energy of the laser beam decreases. Different metalsmay have different SPR wavelength. Therefore, when two metals havingdifferent SPR wavelengths are irradiated by a laser beam having awavelength consistent with the SPR wavelength of one metal, the onemetal may absorb most of the laser energy and melt. The other metal mayonly absorb a very small amount of the laser energy, or may essentiallyabsorb no laser energy, and thus is essentially unchanged by the laserbeam.

For example, silver (Ag) and copper (Cu) have different SPR wavelengths.As an example, for Ag nano particles having a size equal to or largerthan about 25 nm, the SPR wavelength may be about 405 nm. Thefull-width-half-height (FWHM) of an absorption spectrum of such Ag nanoparticles may be about 126 nm, with a wavelength range being from about363 nm to about 489 nm. As another example, for Ag nano particles havinga size equal to or smaller than about 20 nm, the SPR wavelength may beabout 400 nm. The FWHM of an absorption spectrum of such Ag nanoparticles may be about 65 nm, with a wavelength range being from about375 nm to about 440 nm. Similarly, as a further example, for Cu nanoparticles having a size between about 50 nm to about 200 nm, the SPRwavelength may be about 600 nm. The FWHM of an absorption spectrum ofsuch Cu nano particles may be about 192 nm, with a wavelength rangebeing from about 557 nm to about 750 nm.

Therefore, in some embodiments consistent with the present disclosure,the first nano particles 1042 and the second nano particles 1062 may beAg nano particles and Cu nano particles, respectively, and the laserbeam 108 may have a wavelength close to the SPR wavelength of the Agnano particles and significantly different from the SPR wavelength ofthe Cu nano particles. In some embodiments, the SPR wavelength of Agnano particles may be from about 390 nm to about 410 nm. The SPRwavelength of Cu nano particles may be from about 590 nm to about 610nm. In some embodiments, the SPR wavelength of Ag nano particles may beabout 410 nm, and the SPR wavelength of Cu nano particles may be about590 nm. In some embodiments, the wavelength of the laser beam 108 may bewithin a range from about 400 nm to about 410 nm. In some embodiments,the wavelength of the laser beam 108 may be about 405 nm. In someembodiments, the laser beam 108 may be generated by, for example, asemiconductor laser, such as a GaN-based semiconductor laser.

In some embodiments, the first nano particles 1042 and the second nanoparticles 1062 may be gold (Au) and Cu, respectively, and the laser beam108 may have a wavelength close to the SPR wavelength of the Au nanoparticles and significantly different from the SPR wavelength of the Cunano particles. The SPR wavelength of Au nano particles may be fromabout 470 nm to about 520 nm. In these embodiments, the wavelength ofthe laser beam 108 may be chosen to be within a range from about 530 nmto about 540 nm. For example, in some embodiments, the wavelength of thelaser beam 108 may be about 532 nm.

The SPR wavelength of nano particles may not only depend on the materialforming the nano particles, but may also depend on the size of the nanoparticles. For example, for copper nano particles having differentsizes, the SPR wavelengths may be different. Therefore, in someembodiments of the present disclosure, the first nano particles 1042 andthe second nano particles 1062 may be formed of the same material, suchas copper, but may have different sizes. In such embodiments, forexample, the size of the copper nano particles in the firstnano-particle layer 104 may be from about 10 nm to about 30 nm, whilethe size of the copper nano particles in the second nano-particle layer106 may be from about 50 nm to about 300 nm. In these embodiments, thewavelength of the laser beam 108 may be chosen to correspond to one ofthe two kinds of copper nano particles having different sizes. Forexample, the wavelength of the laser beam 108 may be from about 590 nmto about 595 nm. As an example, the wavelength of the laser beam 108 maybe about 593 nm.

The first and second nano particles 1042 and 1062 may be coated with aprotective agent, to prevent aggregation of the nano particles. Fordifferent nano particles, the protective agent may be different. Forexample, for Ag nano particles, the protective agent may bepolyvinylpyrrolidone (PVP), polyacrylic acid (PAA), polyethylene glycol,alkyl amine (oleyl amine), or alkyl thiol. For Cu nano particles, theprotective agent may be PVP, alkyl amine (oleyl amine), or alkyl thiol.Nano particles coated with different protective agents may be dissipatedin different solutions. For example, nano particles coated with PAA andpolyethylene glycol may be dissipated in water, while nano particlescoated with alkyl amine (oleyl amine) or alkyl thiol may be dissipatedin toluene or teradecane.

The content of the protective agent coating the nano particles may alsobe different for different nano particle sizes. For example, if the Agnano particles consisting of the first nano particles 1042 have a sizeequal to or larger than about 25 nm, the content of protective agent byweight may be about 4% to about 6%. For another example, if the Ag nanoparticles consisting of the first nano particles 1042 have a size equalto or smaller than about 20 nm, the content of protective agent byweight may be about 8% to about 10%.

FIG. 1D shows that the first nano particles 1042 within regionsirradiated by the laser beam 108 are selectively heated and melted. Themelted first nano particles 1042 in the first nano-particle layer 104solidify to form adhesion layers 1046. Part of the melted first nanoparticles 1042 enters the second nano-particle layer 106 and, aftersolidifying, serves as filling material 1048 filling space between thesecond nano particles 1062, as shown in the enlarged view of FIG. 1D.The filling material 1048 adheres to the second nano particles 1062. Inaddition, the melted first nano particles 1042 may also adhere to eachother and adhere to the substrate 102.

A cleaning solution is then used to remove the unmelted first nanoparticles 1042 and the unadhered second nano particles 1062. Since thesecond nano particles 1062 above the adhesion layers 1046 are adhered bythe filling material 1048, they are not removed by the cleaningsolution. After the treatment by the cleaning solution, the adhesionlayers 1046, the second nano particles 1062 above the adhesion layers1046, and the filling material 1048 are left on the substrate 102,forming micro bumps, as shown in FIG. 1E.

In the embodiments described with respect to FIGS. 1A-1E, the microbumps are formed by irradiating with the laser beam 108 to melt part ofthe first nano particles 1042. The horizontal size of the micro bumpsmay be controlled by controlling a spot size of the laser beam 108projected on the second nano-particle layer 106 to control a size of aregion within which the first nano particles 1042 may melt. In someembodiments, the laser beam 108 may directly irradiate the secondnano-particle layer 106, and the spot size of the laser beam 108 may bechanged by using different laser sources. In some embodiments, the laserbeam 108 may be projected onto the second nano-particle layer 106through a lens. By using different lens or changing a distance betweenthe lens and a top surface of the second nano-particle layer 106, thespot size of the laser beam 108 may be changed. In some embodiments, thespot size of the laser beam 108 may be smaller than about 10 μm, so thatthe as-formed micro bumps may have a horizontal size smaller than about10 μm. In some embodiments, the spot size of the laser beam 108 may belarger than about 5 μm but smaller than about 10 μm, so that theas-formed micro bumps may have a horizontal size larger than about 5 μmbut smaller than about 10 μm. In some embodiments, the spot size of thelaser beam 108 may be about 10 μm, so that the as-formed micro bumps mayhave a horizontal size of about 10 μm. In some embodiments, the spotsize of the laser beam 108 may be about 5 μm, so that the as-formedmicro bumps may have a horizontal size of about 5 μm.

FIGS. 2A-2F schematically show another method for forming micro bumpsconsistent with embodiments of the present disclosure. Referring to FIG.2A, similar to that shown in FIG. 1A, a first nano-particle layer 204consisting of first nano particles 2042 is formed on a substrate 202.The method for forming the nano-particle layer 204 may be the same asthat for forming the nano-particle layer 104, and thus is not describedhere.

Next, as shown in FIG. 2B, the first nano-particle layer 204 ispatterned to form adhesion pads 2044. In some embodiments, the firstnano-particle layer 204 may be patterned by, for example, aphotolithography process.

In some embodiments, bonding layers (not shown), such as copper, gold,silver, or nickel bonding layers, may have been formed on the substrate202 before forming the first nano-particle layer 204. In theseembodiments, the bonding layers may be formed by first forming a bondingmaterial layer and then patterning the bonding material layer using, forexample, a photolithography process. The patterning of the firstnano-particle layer 204 may be performed by a self-alignment processwhich keeps the first nano particles 2042 deposited on the bondinglayers to form the adhesion pads 2044 but removes the first nanoparticles 2042 deposited directly on the substrate 202.

In some embodiments, self-assembly monolayers may be formed on thebonding layers to assist the formation of the adhesion pads 2044.Self-assembly monolayer may improve the adhesion of the first nanoparticles 2042 to the bonding layers. In these embodiments, afterpatterning the bonding material layer to form the bonding layers,self-assembly monolayers may be formed on the bonding layers.

The material suitable for the self-assembly monolayers may have a headgroup and a functional end group. In such a material, a head group maybe connected to an alkyl chain, where one end of the alkyl chain notconnected to the head group may be functionalized (i.e., adding —OH,—NH3, or —COOH groups) to vary wetting and interfacial properties of theself-assembly monolayers. The functional end group may be selected from,but not be limited to, —OH, —CHO, —COOH, —SH, —CONH₂. The type of thehead group depends on the application of the self-assembly monolayers.Surfaces on which head groups may be held may be planar surfaces, suchas surfaces of silicon wafer and metal layers, or curved surfaces, suchas surfaces of nanoparticles. Material suitable for self-assemblymonolayers may be alkanethiols, disulfide, dialkyl disulfides, dialkylsulfide, alkylxanthate, or dialkylthiocarbamate. Alkanethiols aremolecules with an alkyl chain as the back bone, a tail group, and an S—Hhead group. They may be used on noble metal substrates because of thestrong affinity of sulfur for these metals. For example, the materialfor the self-assembly monolayers may be HS—C_(n)H_(2n)—COOH (such asHS—C₃H₆—COOH), HS(CH₂)₁₆OH, HS(CH₂)₁₅CO₂CH₃, HS(CH₂)₁₅CH₃,HS(CH₂)₁₅COOH, HS(CH₂)₁₆SO₄H, HS(CH₂)₉CH₃, NH₂(CH₂)_(n)SH,HS(CH₂)₁₁CONH₂, or Si(OCH₃)₃—(CH₂)_(n)SH. The S—H head group in suchmaterial may form a covalent bond with the metal in the bonding layersso as to adhere the material to the bonding layers. On the other hand,since the S—H head group does not react with the substrate, theself-assembly monolayer material may not adhere to the substrate. Thus,the self-assembly monolayers may be formed on the bonding layers.

After forming the self-assembly monolayers on the bonding layers, nanoparticles 2042 are deposited. Some of the nano particles 2042 aredeposited on the self-assembly monolayers and thus adhered thereto.Other nano particles 2042 not deposited on the self-assembly monolayerscan be removed easily. Alternatively, in some other embodiments,material containing Si(OC_(n)H_(2n+1))_(x) (x=1˜4) may be used as thematerial for the self-assembly monolayers. In these embodiments, thematerial for the self-assembly monolayers may form covalent bonds withsilicon in the substrate so as to attach the material to the siliconsubstrate. In this way, the self-assembly monolayers may be formed onthe exposed silicon substrate, rather than on the metal bonding layers.After that, the metal bonding layers may be removed and nano particles2042 be deposited.

In some embodiments, the substrate 202 may be patterned to form aplurality of protruding portions before forming the first nano-particlelayer 204. First nano particles 2042 may be deposited on and adhere toupper surfaces of the protruding portions of the substrate 202, formingthe adhesion pads 2044 directly.

In some embodiments, the adhesion pads 2044 may have a horizontal sizesmaller than 10 μm.

After the adhesion pads 2044 are formed, a second nano-particle layer206 consisting of second nano-particles 2062 is formed over the adhesionpads 2044 and the substrate 202, as shown in FIG. 2C. The method forforming the nano-particle layer 206 may be the same as that for formingthe nano-particle layer 106, and thus is not described here.

Referring to FIG. 2D, a laser beam 208 is then irradiated onto thesecond nano-particle layer 206 to selectively heat and melt the firstnano particles 2042 in the adhesion pads 2044. In the process shown inFIG. 1C, since the horizontal size of the micro bumps is determined bythe spot size of the laser beam 108, the spot size of the laser beam 108may need to be controlled to be smaller than 10 μm to achieve microbumps having horizontal size smaller than 10 μm. However, in the processshown in FIG. 2D, the horizontal size of the micro bumps is determinedby the horizontal size of the adhesion pads 2044. Thus, it may not benecessary to control the spot size of the laser beam 208 to be smallerthan 10 μm to achieve micro bumps having a horizontal size smaller than10 μm. That is, the spot size of the laser beam 208 may be larger thanthe desired horizontal size of the micro bumps, and thus a spot of thelaser beam 208 projected on the second nano-particle layer 206 may covera plurality of adhesion pads 2044. Therefore, in the process shown inFIG. 2D, a laser beam having a relatively large diameter may be used andit may not be necessary to use a lens to converge the laser beam. As aresult, the cost for generating the laser beam 208 may be reduced. Forillustrative purposes, in FIG. 2D, the symbol for the laser beam 208 isshown to be larger than the symbol for the laser beam 108 in FIG. 10.This schematically indicates that the laser beam 208 may have a diameterlarger than the horizontal size of the laser beam 108 in FIG. 1C.

With reference to FIG. 2E, after the irradiation by the laser beam 208,the melted first nano particles 2042 in the adhesion pads 2044 solidifyto form adhesion layers 2046. Part of the melted first nano particles2042 enters into the second nano-particle layer 206 and, aftersolidifying, serves as filling material 2048 filling the space betweenthe second nano particles 2062, as shown in the enlarged view of FIG.2E, The filling material 2048 adheres to the second nano particles 2062above each adhesion layer 2046.

A cleaning solution is then applied to remove the unadhered second nanoparticles 2062. Thus, the adhesion layers 2046, the second nanoparticles 2062 above the adhesion layers 2046, and the filling material2048 are left on the substrate 202, forming micro bumps, as shown inFIG. 2F.

In the method shown in FIG. 2A-2F, the first nano particles 2042 and thesecond nano particles 2062 may be the same as the first nano particles1042 and the second nano particles 1062 in FIGS. 1A-1E, respectively.The thicknesses of the first nano-particle layer 204 and the secondnano-particle layer 206 may also be the same as those of the firstnano-particle layer 104 and the second nano-particle layer 106 in FIGS.1A-1E, respectively. Therefore, description of properties of the firstnano particles 2042, the second nano particles 2062, the firstnano-particle layer 204, and the second nano-particle layer 206 isomitted.

FIGS. 3A and 3B schematically show a chip having a substrate 302 and aplurality of micro bumps 310 formed on the substrate 302 using methodsconsistent with embodiments of the present disclosure. FIG. 3A is across-sectional view and FIG. 3B is a plan view. The structure shown inFIG. 3A is similar to those shown in FIG. 1E and FIG. 2F. In the planview of FIG. 3B, the micro bumps 310 are illustrated to have a circularshape. In other embodiments, the micro bumps 310 may have another shapein the plan view, such as a square shape or a rectangular shape.

Consistent with embodiments of the present disclosure, the micro bump310 includes an adhesion layer 3046 formed of a first metal material.The adhesion layer 3046 may have a thickness of, for example, about 100nm to about 1000 nm. A bump layer 312 is formed on the adhesion layer3046. The bump layer 312 includes a plurality of nano particles 3062formed of a second metal material. The nano particles 3062 may have asize, such as diameter, of, for example, about 20 nm to about 300 nm.The bump layer 312 further includes a filling material 3048 formed ofthe first metal material. In these embodiments, the first metal materialand the second metal material may be different metals. The fillingmaterial 3048 fills a space between the nano particles 3062 and adhereto the nano particles 3062. In some embodiments, the filling material3048 may fill the space between the nano particles 3062 in a manner suchthat there is essentially no void formed in the bump layer 312.

Consistent with embodiments of the present disclosure, a weight ratio ofthe filling material 3048 to the nano particles 3062 may decrease froman interface between the bump layer 312 and the adhesion layer 3046 to atop surface of the bump layer 312. In some embodiments, this ratio maybe about 150:1 to about 10:1 near an interface between the bump layer312 and the adhesion layer 3046. In some embodiments, this ratio may beabout 100:1 to about 20:1 near an interface between the bump layer 312and the adhesion layer 3046. In some embodiments, this ratio maydecrease to about 1:1 to about 1:20 near a top surface of the bump layer312. For example, the weight ratio of the filling material 3048 to thenano particles 3062 may be about 1:20 near the top surface of the bumplayer 312. The change of weight ratio is primarily due to acorresponding change in the packing density of the nano particles.

In some embodiments, an amount of the filling material 3048 may decreasefrom the interface between the bump layer 312 and the adhesion layer3046 to the top surface of the bump layer 312.

Consistent with embodiments of the present disclosure, the first andsecond metal materials may be different metals. For example, the firstmetal material may be silver and the second metal material may becopper. In some embodiments, the first and second metal materials may bethe same metal, such as copper. However, as previously explained, whenthe first and second metal materials are the same, a size of nanoparticles used to form the filling material 3048 may be different from asize of the nano particles 3062.

FIG. 4 schematically shows a package formed by bonding two integratedcircuit chips in a face-to-face manner using micro bumps consistent withembodiments of the present disclosure. The package comprises a firstsubstrate 402 a of a first chip and a second substrate 402 b of a secondchip. The first substrate 402 a and the second substrate 402 b includefirst electrodes 4022 a and second electrodes 4022 b, respectively. Onthe first electrodes 4022 a and the second electrodes 4022 b are formedfirst adhesion layers 4046 a and second adhesion layers 4046 b,respectively, The first adhesion layers 4046 a and the second adhesionlayers 4046 b are formed of a first metal material. Bump layers 412 areformed between the first adhesion layers 4046 a and the second adhesionlayers 4046 b.

Consistent with embodiments of the present disclosure, the bump layers412 may include a bump material as a major component of the bump layers412. The bump material may be formed of a second metal material. Thebump layers 412 may further include a filling material formed of thefirst metal material. The filling material may fill a space in the bumpmaterial and adhere to the bump material. In some embodiments, thefilling material and the bump material may be mixed in a manner suchthat there is essentially no void formed in the bump layers 412.

Consistent with embodiments of the present disclosure, a weight ratio ofthe filling material to the bump material may decrease from an interfacebetween the bump layers 412 and the adhesion layers 4046 a to a middleof the bump layers 412, and increases from the middle of the bump layers412 to an interface between the bump layers 412 and the adhesion layers4046 b. As previously explained, the change in weight ratio may beprimarily due to a corresponding change in the packing density of thenano particles.

In some embodiments, an amount of the filling material may decrease fromthe interface between the bump layers 412 and the adhesion layers 4046 ato a middle of the bump layers 412, and increases from the middle of thebump layers 412 to the interface between the bump layers 412 and theadhesion layers 4046 b.

In the package shown in FIG. 4, both chips have micro bumps formed ontheir respective substrates. Consistent with embodiments of the presentdisclosure, a package may also be formed by bonding one chip havingmicro bumps to another chip without micro bumps. For example, the microbumps of one chip may be bonded directly to bonding layers of anotherchip. In other embodiments, a package may also be formed in aface-to-back manner, in which micro bumps on one substrate may bedirectly bonded to a back surface of another substrate, and thenelectrically connected to circuits on a top surface of the anothersubstrate via through holes, such as TSVs.

Other embodiments of the present disclosure will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

What is claimed is:
 1. A method for forming a micro bump, comprising:forming a first nano-particle layer on a substrate, the firstnano-particle layer including a plurality of first nano particles;forming a second nano-particle layer on the first nano-particle layer,the second nano-particle layer including a plurality of second nanoparticles; and irradiating a laser beam onto the second nano-particlelayer, the laser beam penetrating through the second nano-particle layerand being at least partially absorbed by at least some of the first nanoparticles to melt at least a portion of the first nano particles, onesof the melted first nano particles adhering to ones of the second nanoparticles, wherein the first nano particles and the second nanoparticles have different absorption rates with respect to the laserbeam.
 2. The method according to claim 1, further including providingthe first nano particles and the second nano particles having differentsurface plasmon resonance (SPR) wavelengths, wherein the irradiatingincludes irradiating the laser beam with a wavelength close to the SPRwavelength of the first nano particles.
 3. The method according to claim1, wherein forming the first nano-particle layer includes forming thefirst nano-particle layer to a thickness of about 100 nm to about 1000nm.
 4. The method according to claim 1, further comprising: removingunmelted first nano particles and unadhered second nano particles. 5.The method according to claim 1, wherein forming the first nano-particlelayer and the second nano-particle layer includes forming the firstnano-particle layer and the second nano-particle layer with the firstnano particles and the second nano particles formed of different metals.6. The method according to claim 5, wherein forming the firstnano-particle layer includes forming the first nano-particle layer withthe first nano particles formed of silver or gold, and wherein formingthe second nano-particle layer includes forming the second nano-particlelayer with the second nano particles formed of copper.
 7. The methodaccording to claim 1, wherein forming the first nano-particle layer andthe second nano-particle layer includes forming the first nano-particlelayer and the second nano-particle layer with the first nano particlesand the second nano particles formed of a same metal, and wherein thefirst nano particles and the second nano particles have different sizes.8. The method according to claim 7, wherein forming the firstnano-particle layer and the second nano-particle layer includes formingthe first nano-particle layer and the second nano-particle layer withthe first nano particles and the second nano particles formed of copper.9. The method according to claim 1, wherein forming the secondnano-particle layer further includes forming the second nano-particlelayer including the second nano particles and a plurality of third nanoparticles, and wherein the third nano particles are same nano particlesas the first nano particles.
 10. A method for forming a micro bump,comprising: forming a first nano-particle layer on a substrate, thefirst nano-particle layer including a plurality of first nano particles;patterning the first nano-particle layer to form a plurality of adhesionpads; forming a second nano-particle layer directly on the adhesion padsand over the substrate, the second nano-particle layer including aplurality of second nano particles; and irradiating a laser beam ontothe second nano-particle layer, the laser beam penetrating the secondnano-particle layer and being at least partially absorbed by at leastsome of the first nano particles in the adhesion pads to melt at least aportion of the first nano particles, ones of the melted first nanoparticles adhering to ones of the second nano particles, wherein thefirst nano particles and the second nano particles have differentabsorption rates with respect to the laser beam.
 11. The methodaccording to claim 10, further comprising: removing unadhered secondnano particles.